Tuned amplifier circuit arrangement



Dec. 15, 1942. E. L. c. WHITE ETAL 2,304,978

TUNED AMPLIFIER CIRCUIT ARRANGEMENT Filed July 2, 1941 ATTORNE ime Dec,s, 1942 7 .TUNED 1 ABRAN e o ow, era to Electric & Musical, IndustriesLimited,

gay, Mlddlesex, Engiandm company or Great Application July 2, 1941,

Serial m ments I In Great Britain June a. 194 '2 Claims. (01. 119-111)-This-invention relates to tuned amplifier circuit arrangements, and isconcerned with methods of varying the frequency'band-width of tunedcircuits coupled to amplifying valves.

Variation of the frequency band-width of an amplifier may be effected byusing highly selective coils in the tuned circuits and reducing theirselectivity by, for example, resistances connected greater detail by wayofexainplewithlreference in shunt with. the coils. Alternatively, coilsof a medium selectivity may be used and positive feedback employed toincrease their selectivity.

. The first-mentioned method is expensive owing I "to the fact that thecoils have to be large and consequently large screening cans must beused. Again, owing to considerations of the space occupied, anadditional limit is imposed upon th size of the coils. Further,compensation has to be effected for los'sof gain due to the lowering ofthe shunt impedance of the tuned circuit caused by the connection of theshunt resistances. Such compensation involves, for example, the gaugingof two resistances, namely, the tuned circuit shunt resistance and aresistance from which the gain compensating potential is derived.

The second-mentioned method, namely, that in which coils of mediumselectivity are employed involves reduction of stability' and thepositive feedback produces increase of gain with increase inselectivity. In such a case, in order that the gain may be keptconstant, a method of tapping down. the tuned circuit would be required,the

input to the amplifier being taken from a selected tapping Point.

The object of the present invention'is to provide amethod of effectingvariation of the bandwidth of an amplifier in which the disadvantages Yin the second-mentioned method are overcome.

According to the present invention in an amplifler circuit arrangementincluding a tuned circuit designed to respond to a predetermined band offrequencies, provision'is made to increase the selectivity of said tunedcircuit by positive feed-' back and in order to counteract the increasedsensitivity due to said positive feedback, further provision is made tointroduce a desired amount of negative feedback a single control elementwhich serves to determine the degree of positive feedback being soarranged in the circuit that as the selectivity is varied the gain atresonance of said tuned circuit remains substantially constant.

In order that the nature of the invention may be more clearly understoodand readily carried into effect, one form of circuit arrangementembodying the invention will now be described in 5 resistance R2.

to the accompanying drawing. a

.Referring to' the drawing. it is'assum'ed that point I). The circuit ofthe valve'V includes the usual grid condenser: e.'.leak resistance fandgrid biasing resistance and condenser combination 'xy. The cathode leadof the valve also includes a variable non-inductive resistance It:whichcontrols the effective band-widthgof the circuit LC"- and'a furthernon-inductive resistance Re iscon nected between the cathode andtheitappingjpoint b asshown. Theresistance B: provides a source ofpositive feedback potential as shown, whichiis transferred to thecontrolfgrid through the .i'e-

sistance R3, butthere will be a simultaneous ef-. 'fectivef reduction ofvalve slopedueito the cathode appropriate value of Ra. Itcan be shownthat if: l 1 1 T1 1T1Z Ts where-R1 is equal to-the eflectiv shuntresistance of the tuned circuit alone, then, is beingthe 1 anodecurrent, the current amplification of the valve at resonance is v M1 7 Iwhich is independent of R2, but the sharpness of resonance, or Q"increases-with increase in the value of R2, the effective negativresistance across the tuned circuit being given by:

a test 'on a particular circuit at a frequency of 18 kcs. per second,a'tuned circuit with a.Q. of 100 gave a variation of eflective'YQ. I asthe resistance R: was varied from 0-5000 ohms, from a 55-530. The changein gainv over this range was 8 per cent. In the condition of highselectivity ahighim- The net effect on'overali gain'of f these twoopposing factors is made zero by'the.

stability was such that an increase of 30 per cent in the voltageapplied to the screening grid of valve V caused no measurable change ingain and a change in selectivity of 0.7 per cent.

It will be understood that the invention may find general application intuned amplifier circuits and one particular use may be found in theintermediate frequency stages of a superh eteroconnecting said cathodeto the low potential end I of said coil, said resistor being traversedby the space current of said tube, an output circuit con- 7 nected tosaid output electrode, a resistive impedance connecting the cathode endof said resister to a point on said coil whose potential is intermediatethe potentials of the ends of said coil, and said unbypassed resistorbeing adjustable in value thereby to control the selectivity of saidtuned input circuit while maintaining the gain at resonance thereofsubstantially constant.

2. In combination with an anti-resonant input circuit tuned to anoperating carrier frequency, an amplifier tube having at least acathode, control grid and output electrode, an output circuit coupled tosaid output electrode, means connecting the control grid to a highpotential point of said input circuit, a resistive impedance connectingsaid cathode to a second point of said input circuit, said second pointhaving a potential whose magnitude is intermediate that of said highpotential point and the input circuit low potential side, an unby-passedresistor in the space current path of said tube and connected between thcathode end of said impedance and the low potential side of the inputcircuit, and means for adjusting the resistive magnitude or saidresistor thereby simultaneously altering in opposite senses theimpedance of said input circuit and the rate of change of outputelectrode current with respect to voltage across the input circuit. ERICLAWRENCE CASLING WHITE. ERIC WILLIAM BULL.

